The invention relates to a method and apparatus for tolerance compensation in matrix printing devices wherein a start of a print clock generation is initiated by a criterion generated at a line start, said print clock generation being dependent upon a scanning clock generated by sensing motion of the printer carriage.
Print heads of dot-matrix printers, as is known, comprise print elements such as needles which are positioned in one or in a plurality of rows. The drive of the individual print elements with information contained in a character generator respectively occurs column-wise, i.e. a column or print clock is supplied to the print head after the print head has moved by a column spacing. The print clock thus defines the point in time at which the print elements are actuated. Every character can be constructed in grid-like fashion on the recording medium in this manner.
As is generally known, the print events always occur during the movement of the print head with constant velocity. The formation of the print clock can thus occur by means of sensing a so-called printer carriage carrying the print head or by sensing the drive means driving the printer carriage. In detail, this occurs such that a sensing system coupled to the drive means, for example a timing disk, is opto-electronically scanned and the scan clocks thus formed are supplied to a control clock generator, for example a PLL circuit, in which a so-called higher-frequency control clock is formed. This is available to a print clock generator which then generates the print clocks. Control and print clocks are respectively generated only during the actual printing operation, i.e. no printing occurs during the acceleration and during the braking of the printer carriage. For this purpose, a criterion formed at the line beginning is interpreted, and which is produced for example by actuation of the so-called line start contact. This criterion, which is generated for example by closing the line start contact at the beginning of a printing line, can be exploited in order to switch on the control clock generator, so that this starts to work with a defined edge of the next scan clock.
The following problem thus occurs. Due to influence of the scanning system (for example, belt dilation, tolerance of the timing disk, etc.) which cannot be avoided or can only be avoided with extremely high expense or due to a functioning of the line start contact that is not absolutely constant, the line start contact can, as viewed chronologically, be actuated in the immediate proximity of a defined edge of the scan clock. That means that the turn-on criterion for the control clock generator which is conditioned by this appears shortly before the defined edge in one instance and shortly after the defined edge of the scan clock at some other time. That leads to a shift of the control clocks, and thus to a shift of the print clocks. The start of printing can thus occur too late or too early by one period of the scan clock. In the final analysis, this leads to a disruptive print offset or mismatch at the line beginning and to a very unsteady print image.